This invention belongs to the field of Air Traffic Control and of Surveillance Systems. In particular it is applied in Cooperating Surveillance SSR (Secondary Surveillance Radar) Mode S and/or Mode A/C Systems and utilises xe2x80x9cMode S Squitterxe2x80x9d.
Mode S squitter require that an aircraft emits Mode S replies spontaneous or on calling (replies to interrogations). These replies are generated from on board equipment called transponder. In this manner ground station (attive or passive) or others aircraft acquire positions and others information about the aircraft.
Present-day Mode S-Receivers decode replies if they do not suffer from interference with other replies. The Mode S replies can be decoded only if they are free from interference or at most, interfered with one Mode A/C reply. No Mode S reply with Mode S interference from other Mode S replies can be recognised and corrected, before the present invention.
This invention goes beyond the present limitations in the capability to recognise and decode Mode S/Mode S interference by suitably using and analysing the replies and the spontaneous replies (xe2x80x9csquitterxe2x80x9d) in the frequency domain besides the traditional processing in the time domain, the latter being the unique analysis used before this invention. The frequency analysis can be used because each transponder produce a central frequency (carrier frequency of the reply) that can be quite different from the others transponders due to the calibrations of the generator, inside a tolerance window defined by international regulations (ICAO standards). A sharp analysis (high resolution) of the frequencies allows the discrimination of superimposed replies that one cannot obtain with time analysis only.
Model based or xe2x80x9csuper resolutionxe2x80x9d methods allow a high-resolution frequency analysis. The present invention preferably uses an algorithm based on the paper of Tufts-Kumaresan, xe2x80x9cEstimation of Frequencies of Multiple Sinusoids: Making Linear Prediction Perform Like Maximum Likelihoodxe2x80x9d Proceeding of IEEE, vol 70, no. 9, September 1982, pp. 975-981.
It is a specific object of the present invention a method for discriminating superimposed Secondary Surveillance Radar (SSR) replies in a digitalized received signal, a reply comprising a preamble and a data-block, the method being characterised in that it comprises the following step:
detecting a start time of one or more replies in said digitalized received signal,
for each detected start time, determining an end time of the corresponding reply,
generating time windows in at least a portion of the digitalized received signal,
performing a spectrum super resolution method to the digitalized received signal in the generated time windows, in order to estimate one or more carrier frequencies in each generated time window,
estimating the number of replies and the relative carrier frequencies in the digitalized received signal, and
reconstructing said one or more replies.
Preferably according to the invention, said digitalized received signal comprises a Log(Sigma) component signal of a digitalized antenna signal.
Still according to the invention, the method may be characterised in that the steps of detecting a start time and determining an end time of one or more replies comprise an envelope detection of the digitalized received signal;
in that the time windows are generated in the whole signal and are partially superimposed;
in that the step of performing a spectrum super resolution method further estimate the number of carrier frequencies in each generated time window;
in that the step of estimating the number of replies and the relative carrier frequencies in the digitalized received signal is carried out through using the estimations obtained in the step of performing a spectrum super resolution method, wherein one or more power thresholds are created through which power in each generated time window is able to be associated with a pertaining reply; and
in that the step of reconstructing said one or more replies estimates, for each reply in each generated time window, the presence and position of reply pulses through using said created one or more power thresholds, wherein the pertaining reply pulses are the ones having the carrier frequencies closest to estimated value, a signal being created for each reply, said signal being re-timed in such a way that the preamble is first re-timed and the data-block is re-timed afterward.
Always according to the invention, the method may be characterised
in that the steps of detecting a start time of one or more replies and estimating the number of replies and the relative carrier frequencies in the digitalized received signal comprise the following sub-step
estimating the arrival time of a leading reply, by using a power monitoring, detecting four preamble pulses,
calculating the timing of the reply, through analysing the detected power levels and the timing of the detected pulses of the preamble,
estimating the carrier frequency through performing a spectrum super resolution method onto said four preamble pulses, in order to obtaining a frequency evaluation for each of said four pulses, and averaging the four evaluations;
in that the time windows are generated according with the timing of the leading reply, corresponding to the chip time on which the pulses of the reply are detectable; and
in that the step of reconstructing said one or more replies estimates comprises a first frequency testing, in order to verify in which of two temporal windows, related to a same bit of the reply, the frequency found for the leading reply is, through deciding for the time window wherein the same frequency of or the frequency value closest to the estimated frequency of the preamble is found.
Furthermore according to the invention, the sub-step of estimating the carrier frequency may perform a spectrum super resolution method wherein
the signal in the pulses is represented by a number of complex sinusoids with unknown amplitudes and frequencies and white additive noise;
a linear prediction filter is defined and used to calculate the unknown parameters of the signal;
eigenvalues and eigenvectors are used to calculate the transfer function of the error prediction filter defined;
on the basis of the eigenvalues, a signal subspace and a noise subspace are defined with the related eigenvectors; and
the zeros on the unitary circle of the filter transfer function, that correspond to the frequencies of the complex sinusoids, are calculated.
It is still an object of the present invention a Secondary Surveillance Radar (SSR) receiving and processing apparatus characterised in that it is adapted to carry out the aforementioned method for discriminating superimposed SSR replies in a digitalized received signal, the apparatus being further characterised in that it comprises:
means for detecting a start time of one or more replies in said digitalized received signal, said means receiving at an input said digitalized received signal;
means for determining, for each detected start time, an end time of the corresponding reply;
means for generating time windows in at least a portion of the digitalized received signal;
means for performing a spectrum super resolution method onto the digitalized received signal in the generated time windows, in order to estimate one or more carrier frequencies in each generated time window, said means receiving at a first input said digitalized received signal and being connected to said means for generating time windows;
means for estimating the number of replies and the relative carrier frequencies in the digitalized received signal, and
means for reconstructing said one or more replies, such means being connected to said means for estimating the number of replies and the relative carrier frequencies.
Still according to the invention, the means for detecting a start time of one or more replies in said digitalized received signal may also receive an expected reply time.
Always according to the invention, the apparatus may be further characterised in that
said means for detecting a start time of one or more replies in said digitalized received signal comprises envelope detecting means adapted to detect the start time, when signal power is higher than a threshold, and the end time of said one or more replies;
said means for estimating the number of replies and the relative carrier frequencies comprise carrier frequency estimating means for estimating, on the basis of the evaluation for each window, the number of replies and the relative carrier frequencies in the digitalized received signal and for creating one or more power thresholds; and
said means for reconstructing said one or more replies comprise re-timing and reply reconstruction means for estimating, for each reply in each generated time window, the presence and position of reply pulses through using said created one or more thresholds, wherein the pertaining reply pulses are the ones having the carrier frequencies closest to estimated value, the re-timing and reply reconstruction means creating a signal for each reply and re-timing the same in such a way that the preamble is first re-timed and the data-block is re-timed afterward.
said re-timing and reply reconstruction means being connected to means for performing a spectrum super resolution method onto the digitalized received signal.
Still according to the invention, the apparatus may be further characterised in that
said means for detecting a start time of one or more replies and said means for estimating the number of replies and the relative carrier frequencies in said digitalized received signal comprise preamble analysing means including
preamble detecting means for estimating the arrival time of a leading reply, by using a power monitoring, and detecting four preamble pulses, start and end detecting means for calculating the timing of the reply, through analysing the detected power levels and the timing of the detected pulses of the preamble, said start and end detecting means being connected to said preamble detecting means, and
carrier frequency estimating means for estimating the carrier frequency through performing a spectrum super resolution method onto said four preamble pulses, in order to obtaining a frequency evaluation for each of said four pulses, and averaging the four evaluations, said carrier frequency estimating means being connected to said preamble detecting means and to said start and end detecting means;
said means for reconstructing said one or more replies includes
frequency testing means for verifying in which of two temporal windows, related to a same bit of the reply, the frequency found for the leading reply is, through deciding for the time window wherein the same frequency of or the frequency value closest to the estimated frequency of the preamble is found, said frequency testing means being connected to said carrier frequency estimating means, to said means for generating time windows and to said means for performing a spectrum super resolution method, and reply reconstruction means for reconstructing said one or more replies, said reply reconstruction means being connected to said frequency testing means.
According to the invention, it is provided a super resolution Processor/Receiver to discriminate superimposed Secondary Surveillance Radar (SSR) replies and squitter with the ability to discriminate and decode superimposed replies using both time and frequency analysis.
The Processor/Receiver may be characterised in that it utilises super resolution, that is exceeds the classic resolution limit (Raylegh limit) using non linear systems and model analysis.
The Processor/Receiver may still be characterised in that estimates the information about the replies carriers and estimate the replies timing using the time and frequency analysis with super resolution techniques of the whole (or part) of the signals coming from one or more replies.
The Processor/Receiver according to the invention may also be characterised in that it estimates the information about the carrier frequencies and timing using an analysis of the preamble of the leading reply with time and spectral super resolution techniques.
The Processor/Receiver according to the invention may furthermore be characterised in that it applies the super resolutions method to time windows where there is a useful signal.
The Processor/Receiver according to the invention may still be characterised in that it uses a super resolution method to study the information bit pulse""s position.
The Processor/Receiver according to the invention may also be characterised in that it applies a super resolution method on the first and on the second part of interval related to each information""s bit to estimate the position of the pulse.
The Processor/Receiver according to the invention may furthermore be characterised in that it uses a super resolution method applied in superimposed time sub windows on the interval in which power is detected.
The Processor/Receiver according to the invention may still be characterised in that it has a reply reconstructor.
The Processor/Receiver according to the invention may also be used for scopes different from Mode S reception. Namely, it can be used in all case in which the pulse coding is used and is necessary to discriminate superimposed signals, both for measurement (E.S.M. Electronic support measurements) or communications scopes.